The present invention relates to a method of oxygen delignification of lignocellulosic material, preferably at medium concentration, i.e., between about 8 and 16%.
Since the original introduction of oxygen delignification operating at medium pulp concentrations not much development work has been devoted to this process. Since the use of chlorine free bleaching and the closing of bleach plants have become matters of immediate interest, extended delignification, i.e., the (a) further lowering of the kappa number by means of oxygen has increasingly become more interesting. Extended delignification by oxygen in one or several steps, however, can result in a deterioration in pulp quality. The use of the appropriate conditions, however, can yield several advantages.
It should thus be possible to maintain the yield of the pulp at a higher level than is the case with extended cooking, i.e., cooking to lower the kappa number.
In a multi-step method it should be possible to distribute the chemicals between the steps in order to obtain optimum conditions in every step. Even other conditions could then be optimized.
In accordance with the present invention, these and other objectives have now been met by the invention of a method for oxygen delignification of a pulp comprising cellulosic material comprising initially delignifying the pulp at an initial delignification temperature of less than 90xc2x0 C. and an initial delignification pressure so as to produce a partially delignified pulp, adding oxygen to the pulp so that the oxygen is present during the initial delignification step, and subsequently delignifying the partially delignified pulp at a subsequent delignification temperature of greater than 90xc2x0 C., and a subsequent delignification pressure, the difference between the initial delignification temperature and the subsequent delignification temperature being less than about 20xc2x0 C., the initial delignification pressure being greater than the subsequent delignification pressure, the method including adding alkali solely to the initial delignification step in order to obtain a high alkalinity therein.
In accordance with one embodiment of the method of the present invention, initial delignifying of the pulp is carried out for an initial residence time of between about 10 and 30 minutes, and subsequent delignifying of the partially delignified pulp is carried out for a subsequent residence time of between about 45 and 180 minutes. Preferably, the initial residence time is between about 15 and 25 minutes and the subsequent residence time is between about 60 and 120 minutes.
In accordance with another embodiment of the method of the present invention, the initial delignifying pressure is between about 4 and 10 bars and the subsequent delignifying pressure is between about 2 and 5 bar.
In accordance with another embodiment of the method of the present invention, the method includes adding the oxygen to the pulp in an amount of between about 25 and 50 kg per ton of pulp.
In accordance with another embodiment of the method of the present invention, the method includes adding the alkali to the initial delignification step in an amount of between about 25 and 50 kg of alkali per tone of pulp.
In accordance with another embodiment of the method of the present invention, the difference between the initial delignification temperature and the subsequent delignification temperature is between about 10 and 15xc2x0 C.
In accordance with another embodiment of the method of the present invention, the method includes adding oxygen to the partially delignified pulp in an amount of up to about 5 kg per ton of pulp.
The present invention thus relates to a method of extended oxygen delignification so that a lower kappa number is obtained without at the same time deteriorating the properties of the pulp. By carrying out extended delignification according to the present invention, the total delignification can amount to between about 50 and 70% of the lignin content (kappa number) of the unbleached pulp. The method is carried out at medium pulp concentrations in two subsequent steps.